Biometric recognition systems are used for authenticating and/or verifying users of devices incorporating the recognition systems. Biometric sensing technology provides a reliable, non-intrusive way to verify individual identity for recognition purposes.
Fingerprints, like various other biometric characteristics, are based on distinctive personal characteristics and, thus, are a reliable mechanism for recognizing an individual. There are many potential applications for utilization of fingerprint sensors. For example, fingerprint sensors may be used to provide access control in stationary applications, such as security checkpoints. Electronic fingerprint sensors may also be used to provide access control in mobile devices, such as cell phones, wearable smart devices (e.g., smart watches and activity trackers), tablet computers, personal data assistants (PDAs), navigation devices, and portable gaming devices. Accordingly, some applications, in particular applications related to mobile devices, may require recognition systems that are both small in size and highly reliable.
Most commercially available fingerprint sensors are based on optical or capacitive sensing technologies. Unfortunately conventional optical fingerprint sensors are too bulky to be packaged in mobile devices and other common consumer electronic devices, confining their use to door access control terminals and similar applications where sensor size is not a restriction. As a result, fingerprint sensors in most mobile devices are capacitive sensors having a sensing array configured to sense ridge and valley features of a fingerprint. Typically, these fingerprint sensors either detect absolute capacitance (sometimes known as “self-capacitance”) or trans-capacitance (sometimes known as “mutual capacitance”). In either case, capacitance at each pixel in the array varies depending on whether a ridge or valley is present, and these variations are electrically detected to form an image of the fingerprint.
While capacitive fingerprint sensors provide certain advantages, most commercially available capacitive fingerprint sensors have difficulty sensing fine ridge and valley features through large distances, requiring the fingerprint to contact a sensing surface that is close to the sensing array. As a result, it remains a significant challenge for a capacitive sensor to detect fingerprints through thick layers, such as the thick cover glass (sometimes referred to herein as a “cover lens”) that protects the display of many smartphones and other mobile devices. To deal with this drawback, a cutout is often formed in the cover glass in an area beside the display, and a discrete capacitive fingerprint sensor (often integrated with a mechanical button) is placed in the cutout area so that it can detect fingerprints without having to sense through the cover glass. Unfortunately, the need for a cutout makes it difficult to form a flush surface on the face of device, detracting from the user experience. Also, the existence of mechanical buttons takes up valuable device real estate.